semaphore implementation

Question

I am getting error in the following program  I want to demonstrate how two processes can share a variable using semaphore  Can anyone guide me   I am not able to debug the errors      include lt stdlib h gt   include lt stdio h gt   include lt unistd h gt   include lt sys ipc h gt   include lt sys sem h gt   include lt semaphore h gt  int main       int pid mutex 1    int semid                   semid of semaphore set      key t key   1234     key to pass to semget        int nsems   1     nsems to pass to semget        semid semget key nsems IPC CREAT 0666     if  semid lt 0            perror  Semaphore creation failed           if   pid   fork     lt  0           perror  fork        return 1        else if pid  0          sem wait  amp semid       printf  IN CHILD PROCESS   n        mutex         printf  value of shared variable   d  mutex       sem post  amp semid       return 0        sem wait  amp semid     printf  IN PARENT PROCESS   n      mutex--    printf  value of shared variable   d  mutex     sem post  amp semid     return 0

User · Answer

Please check this out below sample code for semaphore implementation Lock and unlock          include lt stdio h gt       include lt stdlib h gt       include  lt sys types h gt       include  lt sys ipc h gt       include lt string h gt       include lt malloc h gt       include  lt sys sem h gt      int main                     int key share id num              char  data              int semid              struct sembuf sb  0 -1 0               key ftok      a                if key    -1                         printf   n n Initialization Falied of shared memory  n n                        return 1                            share id shmget key 1024 IPC CREAT 0744               if share id    -1                         printf   n n Error captured while share memory allocation n n                        return 1                            data  char   shmat share id  void   0 0               strcpy data  Testing string n                if  fork        Child Porcess                  sb sem op -1    Lock                  semop share id  struct sembuf    amp sb 1                     strncat data  feeding form child n  20                     sb sem op 1   Unlock                  semop share id  struct sembuf    amp sb 1                     Exit 0                 else         Parent Process               sb sem op -1    Lock               semop share id  struct sembuf    amp sb 1                   strncat data  feeding form parent n  20                  sb sem op 1   Unlock               semop share id  struct sembuf    amp sb 1                              return 0

User · Answer

The fundamental issue with your code is that you mix two APIs  Unfortunately online resources are not great at pointing this out  but there are two semaphore APIs on UNIX-like systems    POSIX IPC API  which is a standard API System V API  which is coming from the old Unix world  but practically available almost all Unix systems   Looking at the code above you used semget   from the System V API and tried to post through sem post   which comes from the POSIX API  It is not possible to mix them   To decide which semaphore API you want you don t have so many great resources  The simple best is the  Unix Network Programming  by Stevens  The section that you probably interested in is in Vol  2   These two APIs are surprisingly different  Both support the textbook style semaphores but there are a few good and bad points in the System V API worth mentioning    it builds on semaphore sets  so once you created an object with semget   that is a set of semaphores rather then a single one the System V API allows you to do atomic operations on these sets  so you can modify or wait for multiple semaphores in a set the SysV API allows you to wait for a semaphore to reach a threshold rather than only being non-zero  waiting for a non-zero threshold is also supported  but my previous sentence implies that the semaphore resources are pretty limited on every unixes  you can check these with the  ipcs  command there is an undo feature of the System V semaphores  so you can make sure that abnormal program termination doesn t leave your semaphores in an undesired state

User · Answer

Your Fundamentals are wrong  the program won t work  so go through the basics and rewrite the program   Some of the corrections you must make are   1  You must make a variable of semaphore type  sem t semvar    2  The functions sem wait    sem post   require the semaphore variable but you are passing the semaphore id  which makes no sense   sem wait  amp semvar        your critical section code sem post  amp semvar     3  You are passing the semaphore to sem wait   and sem post    without initializing it  You must initialize it to 1  in your case  before using it  or you will have a deadlock   ret   semctl  semid  1  SETVAL  sem   if  ret    1       perror  Semaphore failed to initialize      Study the semaphore API s from the man page and go through this example

User · Answer

Vary the consumer-rate and the producer-rate  using sleep   to better understand the operation of code  The code below is the consumer-producer simulation  over a max-limit on container    Code for your reference    include  lt stdio h gt   include  lt pthread h gt   include  lt semaphore h gt   sem t semP  semC  int stock count   0  const int stock max limit 5   void  producer void  arg        int i  sum 0      for  i   0  i  lt  10  i               while stock max limit    stock count               printf  stock overflow  production on wait   n                sem wait  amp semC               printf  production operation continues   n                       sleep 1       production decided here         stock count            printf  P  stock-count    d n  stock count           sem post  amp semP           printf  P  post signal   n              void  consumer void  arg        int i  sum 0      for  i   0  i  lt  10  i               while 0    stock count               printf  stock empty  consumer on wait   n                sem wait  amp semP               printf  consumer operation continues   n                       sleep 2       consumer rate decided here         stock count--          printf  C  stock-count    d n   stock count           sem post  amp semC           printf  C  post signal   n                 int main void         pthread t tid0 tid1      sem init  amp semP  0  0       sem init  amp semC  0  0            pthread create  amp tid0  NULL  consumer  NULL           pthread create  amp tid1  NULL  producer  NULL           pthread join tid0  NULL           pthread join tid1  NULL        sem destroy  amp semC       sem destroy  amp semP        return 0

[c] semaphore implementation

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Examples related to semaphore